Rafael Nadal has always been the one blemish on an otherwise perfect resume for Roger Federer. Nadal's record against Federer is probably the sole reason there is not complete agreement about Federer being the greatest of all time.
Take Nadal out of the equation and Federer would most likely sit with 20+ Slams including those very coveted calendar Slams. Not to mention close to 400 weeks as the world No. 1 and a French Open resume that included not one, but four or five wins.
He would be the male equivalent of Steffi Graf, dominating all surfaces and all Slams.
But Federer never really found a way to deal with the Nadal riddle, especially in their many clay matches. Though Federer did enjoy a stretch of success in the middle of their rivalry, he never really managed to respond to the Nadal challenge.
The rivalry started with Nadal leading 6-1 before Federer won five of the next seven to make it 6-8. Since then, Nadal is 9-2 and 17-8 overall.
While the argument is less often voiced these days, it is still the same—how can Federer be the greatest of all time when he loses twice as often as he wins against his biggest rival or his own era?
While Nadal has forced Federer to become a better tennis player, and vice versa, Nadal has also "tarnished" Federer's legacy—as possible as that is to do against a 16-time Slam winner.
Now, Nadal is faced with the toughest challenge of his career and his ability to respond will determine part of his own legacy.
This may sound strange, as Nadal had to usurp the "greatest player of all time" to become the world No. 1 and win most of his slam trophies.
But Nadal has always known how to beat Federer. His game is tailor-made for it.
Even in what is probably Federer's best year ever, 2006, the Swiss Maestro suffered four of his five defeats (92-5 for the season) at the hands of the Spanish Bull. This includes one of the greatest matches ever played, as Nadal secured the fifth set tiebreaker on Rome's clay.
While Nadal has faced many injuries in his career, it's always been a ride pointing toward the sky. Nadal has never had to face a rival who consistently beat him. He's never had to face a player against whom his game wasn't enough on most days.
Now, such a player has arrived in the form of Novak Djokovic.
Djokovic was 0-5 against Nadal in tournament finals prior to this season, but now he's 6-5. Nadal has had a hard time winning sets over Djokovic and only managed four in their six matches this year.
Djokovic won 14.
As Nadal said after his Wimbledon loss to Djokovic, "[Nadal's] game don't bother [Djovkoic] a lot." Djokovic had solutions for everything Nadal brought to the table and has essentially beat Nadal at his own game.
I've argued that we should not count Rafa out, because he has always shown an ability and willingness to improve, even while at the top.
But I've also argued Djokovic has the upper hand in the rivalry for a reason (see here, among others).
And it is precisely this fact—Djokovic was so consistently better than Nadal this year (6-0 is a very convincing stat)—that presents Nadal with a unique opportunity.
If Nadal can come back from adversity of this kind and regain the upper hand in the rivalry with Djokovic, it will propel his legacy to new heights.
He would, in effect, do what Federer failed to do except for a short stretch—deny and dethrone his (slightly) younger challenger.
To be fair to Federer, he did win five of seven matches against Nadal from Wimbledon 2006 to the end of 2007, and he did regain his No. 1 in 2009. But it's hard to deny that Nadal choosing to withdraw from Wimbledon aided Federer in this respect. He is 2-9 against Rafa since 2008.
Moreover, there's almost a five-year age gap between Federer and Nadal, so it is only natural the older Federer couldn't turn the tables in the rivalry once the younger challenger had caught up with him in the rankings.
While most observers would agree Federer has a genuine matchup problem against Nadal, this is much less noticeable in the battle between Nadal and Djokovic. Rafa beat Djokovic 16 times prior to 2011.
Still, with these caveats in mind, I would say if Nadal starts dominating Djokovic again (while Djokovic is healthy) and takes back his No. 1 ranking, it would represent something Federer couldn't manage to do.
But tennis isn't just about what Federer didn't do and Nadal might do.
More importantly, it would represent Nadal overcoming the biggest hurdle he's had to face. It would mean Nadal solved the puzzle against the man who's beaten him six straight times in finals matches.
More Slam wins would be the natural consequence.
If he succeeds in this quest, we have all the more reason to hail him as one of the fiercest fighters and warriors tennis has ever seen.
His legacy among the all time greats is already secure, but this would further his case.